1. Field of the Invention
The present invention relates to a medical X-ray image processing apparatus which converts an image of X-rays penetrated an object into an electric image signal and indicates the converted signal on a CRT display for example.
2. Description of the Prior Art
A widely used medical X-ray apparatus is a unit which converts an X-ray image into an electric image signal and indicates the converted signal on a CRT display for example, or prints the content of the signal using a printer or records the signal using a storage device as necessary. This kind of unit requires an X-ray sensor which converts an image of X-rays penetrated the affected portion (object) of a patient into an electric signal and a data processor which processes the electric signal to generate an electric image signal corresponding to the X-ray image. Such an X-ray sensor generally comprises a fluorescent substance which converts X-rays into visible light, a solid-state image sensor, such as a charge-coupled device, and a light conducting member, such as an optical fiber or optical lens, which conducts the visible light from the fluorescent substance to the solid-state image sensor. A variety of methods have been proposed to enhance the resolution of the image by improving the X-ray sensor (as disclosed by the U.S. Pat. Nos. 4,593,400 and 4,987,307 for example).
In the case of the above-mentioned X-ray sensor, all the X-rays are not converted into the visible light by the fluorescent substance, but several percent of the X-rays penetrate the fluorescent substance and directly reach the solid-state image sensor. When a charge-coupled device (hereafter referred to as "CCD") is used as a solid-state image sensor, its X-ray responding region has generally very low quantum efficiency at 10 KeV or more and its visible wavelength region is centered at about 550 nm. Since the device is responsive to both X-rays and visible light, the X-rays having directly reached the CCD affect the charges stored in the device. As a result, the obtained electric signal includes the information of the X-rays having directly reached the CCD and the information is superimposed as noise on the true image information derived from the visible light. Accordingly, the image formed by the image signal obtained by processing the electric signal of the CCD is dotted with noise spots in the image mostly based on the visible light information as if foreign matters were present in the affected portion of the patient, thereby reducing the resolution of the image and causing the problem of making diagnosis difficult.
To reduce the X-rays which cause the noise as few as possible, instead of arranging the fluorescent substance and the CCD on a straight line, conventional countermeasures have been taken; the CCD is dislocated by appropriately using an inclined optical fiber, reflector or optical lens, and an optical-fiber including X-ray absorbing material such as lead is used (as disclosed by the Japanese Patent Application Laid-open Publication No. 63-308592 and the U.S. Pat. No. 4,910,405 for example). However, if the CCD is dislocated, the entire size of the sensor must be made larger, thereby making it difficult to obtain a compact X-ray sensor capable of being accommodated in a restricted space in such a case wherein the sensor is used as a dental oral sensor to be inserted into the mouth of the patient for example. Furthermore, such a sensor which uses X-ray absorbing material to discriminate the X-rays and visible light cannot deliver sufficient discrimination performance when used in a restricted space in such a case wherein the sensor is used as an oral sensor.
Moreover, the X-rays being incident on the fluorescent substance always generate secondary X-rays, and the secondary X-rays generated in the shape of a tree by an avalanche effect penetrate the light conducting member and affect the charges stored in the CCD. Although the effect of the secondary X-rays can be reduced by increasing the thickness of the light conducting member, it is difficult to completely eliminate the effect. Besides, since the size of the light conducting member is limited, no desirable effect can be obtained.